The Link Layer

The link layer is the lowest layer of the TCP/IP model; it is also referred to in some texts as the network interface layer. The link layer combines the physical and data link layer functions into a single layer. This includes frame physical network functions like modulation, line coding and bit synchronization, frame synchronization and error detection, and LLC and MAC sublayer functions. Common protocols include the Address Resolution Protocol (ARP), Neighbor Discovery Protocol (NDP), IEEE 802.3 and IEEE 802.11.

The Internet Layer

The Internet layer is the next layer up from the link layer and is associated with the network layer of the OSI model. Functions include traffic routing, traffic control, fragmentation, and logical addressing. Common protocols include IP, ICMP and IGMP.

The Transport Layer

The Transport layer is the next layer and is typically related directly with the same named layer in the OSI model. Functions include message segmentation, acknowledgement, traffic control, session multiplexing, error detection and correction (resends), and message reordering. Common protocols include the Transport Control Protocol (TCP) and User Datagram Protocol (UDP).

The Application Layer

The Application layer is the highest layer in the TCP/IP model and is related to the session, presentation and application layers of the OSI model. The application layer of the TCP/IP model is used to handle all process-to-process communication functions; these functions were carried out by multiple different layers when referencing the OSI model. There are a number of different functions which are carried out by this layer, including session establishment, maintenance and termination, character code translations, data conversion, compression and encryption, remote access, network management and electronic messaging to name a few. Common protocols include Named Pipes, NetBIOS, MIME, TLS, SSL, FTP, DNS, HTTP, SMTP and many others.

The Network Layer is responsible for routing, which is moving packets (the fundamental unit of data transport on modern computer networks) across the network using the most appropriate paths. It also addresses messages and translates logical addresses (i.e., IP addresses) into physical addresses (i.e., MAC addresses).